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Satellite tracking of Swedish Ospreys Pandion haliaetus: Autumn migration routes andorientation

Hake, M; Kjellén, Nils; Alerstam, Thomas

Published in:Journal of Avian Biology

DOI:10.1034/j.1600-048X.2001.320107.x

2001

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Citation for published version (APA):Hake, M., Kjellén, N., & Alerstam, T. (2001). Satellite tracking of Swedish Ospreys Pandion haliaetus: Autumnmigration routes and orientation. Journal of Avian Biology, 32(1), 47-56. https://doi.org/10.1034/j.1600-048X.2001.320107.x

Total number of authors:3

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JOURNAL OF AVIAN BIOLOGY 32: 47–56. Copenhagen 2001

Satellite tracking of Swedish Ospreys Pandion haliaetus : autumnmigration routes and orientation

Mikael Hake, Nils Kjellen and Thomas Alerstam

Hake, M., Kjellen, N. and Alerstam, T. 2001. Satellite tracking of Swedish OspreysPandion haliaetus : autumn migration routes and orientation. – J. Avian Biol. 32:47–56.

Autumn migration routes and orientation of Swedish Ospreys Pandion haliaetus werestudied by satellite tracking of 18 birds. Of these, 13 could be followed during theentire migration (6 females, 5 males and 2 juveniles). Most birds migrated acrosswestern and central Europe to winter in tropical West Africa. However, one juvenileflew to Cameroon and one female used a very easterly route and reached Mozam-bique. On average, the birds travelled a total distance of about 6700 km, with littlevariation except for the female wintering in Mozambique, who travelled more than10000 km. Of 21 stopovers (of \1 day), only five were made south of 45°N; threeof these in Africa. Females departed before males and juveniles and flew to astopover site they probably were familiar with. After 3–4 weeks there, they continuedto their wintering grounds. Also males and juveniles usually made one or morestopovers. Adults seemed to travel to a known wintering site, where they remainedstationary, whereas juveniles were more mobile after reaching tropical regions,probably looking for good wintering sites. Males generally left the breeding area indirections similar to the mean migratory direction, whereas a few females departed indiverging initial directions. Apart from these diversions, adult Ospreys followed verystraight migratory routes, with overall mean directions of 185–209° and with meanangular deviations of 6–33°. Some juveniles also departed in diverging directions.Moreover, young birds tended to show a larger variability in orientation. Thus, theOspreys kept a fairly straight direction and did not avoid geographical obstacles suchas mountain ranges and desert areas. However, they seemed reluctant to cross largewater bodies. There was no correlation between angular deviation and length of themigrational segment, indicating that the principles of orientation by vector summa-tion may not be valid for Osprey migration. Moreover, the geographic direction ofmigration did not vary in accordance with variations in the magnetic declination,suggesting that the Ospreys did not orient along magnetic loxodromes.

Mikael Hake, Grimso Wildlife Research Station, Department of Conser6ation Biology,SLU, SE-730 91 Riddarhyttan, Sweden. E-mail: mikael.hake@n6b.slu.se. Nils Kjellenand Thomas Alerstam, Department of Animal Ecology, Ecology Building, SE-223 62Lund, Sweden.

The Osprey Pandion haliaetus has a wide distributionin the northern hemisphere, but also breeds in thetropics. Most, however, breed in temperate areas,mainly between latitudes 40° and 70°N, and are long-distance migrants to tropical regions (Cramp andSimmons 1980, Poole 1989). Ringing recoveries showthat most Ospreys breeding in northern Europe mi-grate across western and central Europe to winter intropical West Africa, from Mauretania eastwards to

Cameroon (O8 sterlof 1977, Saurola 1995). However,there are also a number of records from East Africaand at least four from South Africa, mainly of birdsringed in Finland, but also of Swedish birds (Saurola1994, Stolt et al. 1998). Moreover, one bird ringed asa chick in northern Norway was found in India dur-ing the winter (NOF 1988), showing that the migra-tion of the species is complex and not fullyunderstood.

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JOURNAL OF AVIAN BIOLOGY 32:1 (2001) 47

Sweden holds a large proportion of the Europeanbreeding population, with an estimated 3200 pairs(SOF 1990), and this population is a potentially impor-tant source of birds for the re-establishment of popula-tions in other parts of Europe, including Scotland(Dennis 1995) and Germany (Schmidt and Kapfer1994). Recently, there has been a tendency towards apopulation decline in some areas in southern Sweden(M. Hake unpubl.). Although this may be due to lowreproductive performance (M. Hake unpubl.), it alsomay be connected to threats Ospreys face during thenon-breeding season, as they spend most of the yearaway from the breeding grounds. Threats may includepollution and acidification of freshwater habitats, aswell as hunting, fishing and fish-farming (e.g. Eriksson1984, Poole 1989, Eriksson and Wallin 1994).

As of 1995, more than 15000 Ospreys have beenringed in Sweden, mainly as chicks. From about 2000ringing recoveries, it seems obvious that the majority ofSwedish Ospreys migrate across western Europe towinter in tropical West Africa (O8 sterlof 1977, Saurola1994, Stolt et al. 1998). However, ringing data providelimited information about actual migration routes orstopover and wintering sites. Moreover, little is knownabout orientation during migration. As the Osprey isless concentrated to migration ‘‘hot-spots’’ than otherspecies (e.g. Alerstam 1990a, Kjellen 1997), it is consid-ered to migrate on a broad front. Therefore, it seemslikely that Ospreys should readily cross geographicalobstacles, such as larger water bodies, deserts andmountain ranges. Although some ringing recoveriessupport this, little is known also about this aspect ofthe Osprey’s migration. Hence, detailed informationabout migration routes and stopover and winteringsites of the Swedish population is important not onlyfor identifying potential risk areas en route, but also toenhance our understanding of the evolution of migra-tion strategies in this species and in birds in general.

During the last decade, satellite telemetry has provedto be a very valuable tool for detailing movements oflarge bird species such as storks (e.g. Berthold et al.1992, 1995) and raptors (e.g. Meyburg et al. 1995a, b).Ospreys have been provided with satellite transmittersin USA as well as in Germany and Sweden. During1995–1996, five and 17 birds were tracked to theirtropical wintering sites from breeding grounds in Ger-many and USA, respectively. However, the results ofthese efforts have not been published (B. Meyburg pers.comm., M. Martell et al. unpubl.). The autumn migra-tion of two Swedish Osprey females fitted with satellitetransmitters during breeding has previously been de-scribed (Kjellen et al. 1997).

In this study, we report on the routes and orientationof 13 Ospreys, including the two mentioned above,migrating between Sweden and tropical Africa, alongwith information on six birds which did not completethe entire migration.

Methods

During 1995–1998, we fitted satellite transmitters on 18Ospreys in an area close to Grimso Wildlife ResearchStation (59°43%N, 15°30%E) in South Central Sweden,about 200 km west of Stockholm. Twelve breedingadults (six of each sex) were caught during the first halfof July, with a clap-net on the nest, or with a net(10×4 m) placed above a Goshawk Accipiter gentilismount close to the nest. The transmitters were attachedas backpacks, and the birds were released within onehour of capture. One of the females (Fe 49–96, seeTable 1), which disappeared in Spain during migrationin September 1998, was followed in both 1996 and 1998(see Table 2). Six juveniles were provided with transmit-ters just before fledging in late July. We used transmit-ters with conventional batteries (Microwave Nano PTT100), weighing 30 g (about 2% of the body weight ofthe birds), on four females, three males and six juve-niles. During 1997–1998, however, two females andthree males were provided with solar-powered transmit-ters, weighing 35 g. Although a few birds apparentlydied during migration, we have no reason to believethat their deaths were caused by the extra load. At leastsix adults subsequently returned to breed in the samenest, and we then recaptured them and removed thetransmitter. After refurbishing, these transmitters werere-used the following seasons. In all cases, the harnesseswere intact and the Ospreys showed no signs of injury.

The transmitters were tracked by CLS/Service Argosin Toulouse, France. The batteries of the 30 g transmit-ters had an expected life-time of at least 500 hours. Thetransmitters were programmed to transmit during 10 hevery third day to cover the autumn migration andparts of the wintering period. Depending on satelliteorbits and local conditions we received 0–15 positionsduring one 10-h period. The solar-powered transmit-ters, on the other hand, were more or less constantlyactive, provided that there was enough sunshine. Al-though these transmitters sometimes failed at northerlylatitudes, they often provided a great number of posi-tions each day.

Locations provided by the Argos system are dividedinto different classes (labelled A, B and 0–3) dependingon validation, number of messages received and loca-tion accuracy. We normally used all validated locationsin this analysis, taking location accuracy into accountonly when relevant (see below). For three classes oflocations (1–3), the accuracy is within 1 km, while theaccuracy for A, B and 0 is unspecified. The high-qualitylocations make up about 10% of our data set. Duringperiods when the Ospreys were stationary within alimited area, e.g. the breeding area, it was obvious thatlocations of unspecified accuracy were often 5–10 kmfrom the correct position, and in extreme cases thedeviation was 20–100 km. However, it seems appropri-ate to use all available locations in the analysis of

48 JOURNAL OF AVIAN BIOLOGY 32:1 (2001)

migratory movements over substantial distances, asthe location error will, in such cases, only affect theresult to a minor degree. In contrast, when consider-ing local movement patterns within limited areas, it isessential to rely primarily on the high-quality loca-tions.

As the transmitters were not normally active everyday, we did not always know the exact date of depar-ture from the breeding grounds and arrival at thewintering sites. If these dates were not known, weused the average speed on travelling days (see Kjellenet al. 2001) to calculate the most likely dates of de-parture and arrival. The total durations of stopoverswere determined by the same method. An area was

considered to be a stopover site if the bird staged forat least 24 h and moved less than 100 km.

Distances and directions are based on standard cal-culations for loxodromes (rhumblines). The total dis-tance migrated was obtained by using a maximum ofone position per 24-h period and adding the dailydistances flown. Furthermore, for plotting, the loca-tions were transformed to coordinates in the Merca-tor projection (Gudmundsson and Alerstam 1998).This map projection has the advantage of showingloxodromes as straight lines, and is therefore tradi-tionally used for nautical charts. However, it is trueneither with respect to distance nor area, exaggeratinggeographical dimensions of high latitudes in com-

Table 1. Autumn migration of thirteen Ospreys followed from the Swedish breeding grounds to Africa during 1995–1998.Approximate coordinates (decimal values) are given for stopover and wintering sites. Indices for the different individuals referto the identification number of the transmitter and the year it was used (e.g. Fe 43–95= female with transmitter no. 43, attachedin 1995).

Wintering site DistanceSex/Age Dates and sites of stopoversMigration periodflown, km(coordinates) (coordinates)

Ind.-Year (average breeders) (average)

655031 Aug–3 Sep W Czech Rep.28 Aug–24 Sep S Ivory Coast (05.2N,Fe 43–9506.2W)(49.8N, 12.8E)

13 Aug–10 Sep W Kazakhstan5 Aug–24 OctFe 44–95(49.3N, 49.8E)27–30 Sep N Ethiopia (12.2N, 37.2E)5–15 Oct Central Uganda 10 060W Mozambique (15.6S,

32.4E)(01.9N, 32.9E)The Gambia (13.5N, 15.8W) 581021–30 Aug NE Germany15 Aug–15 SepFe 48–96

(About 51.0N, 7.5E)Fe 49–96 22 Aug–27 Sep 25 Aug–5 Sep SW Norway 5980W Senegal (14.8N, 17.3W)

(58.9N, 06.1E)Fe 51–97 28 Jul–17 Sep 2–29 Aug N Germany (53.5N,10.7E) S Senegal (12.9N, 16.7W) 6090Fe 49–98 7 Aug–30 Sep 9–13 Aug SE Sweden

(About 57.0N, 16.0E)15–16 Aug NE Germany (53.9N, 13.3E)

691020 Aug–12 Sep E Austria (47.8N, 17.5E) E Guinea Bissau (12.1N,14.5W)

(6900)(12 Aug–27 Sep,Total Fen=6)

– 60404–17 SepMa 50–96 Guinea Bissau (11.5N,16.0W)

3 Aug–26 SepMa 55–97 10 Aug–1 Sep E France (48.2N, 04.5E)3–12 Sep Central France (46.8N, 03.8E) SW Mali (11.5N, 08.3W) 593019 Sep–2 Oct W Poland (51.1N, 15.6E)12 Sep–5 NovMa 51–985–8 Oct Central Austria (47.2N, 13.3E)

7230Ivory Coast (07.3N, 05.5W)19–25 Oct N Tunisia (36.0N Oct. 5E)5–6 Sep N Germany (54.8N, 11.5E)Ma 52–98 3 Sep–18 Oct10–14 Sep E Germany (51.6N, 14.0E)16–18 Sep W Czech Rep. (49.7N, 13.4E)23–30 Sep S France (43.8N, 01.3E) The Gambia (13.6N, 16.5W) 6570

6320W Sierra Leone17 Sep–29 Oct 23 Sep–6 Oct N Germany (52.0N, 10.7E)Ma 54–98(08.8N, 13.2W)

(9 Sep–13 Oct,Total Ma (6420)n=4)

7270Juv 53–96 Central Guinea15 Sep–6 Oct S Spain (36.4N, 06.0E)3 Sep–26 Oct(11.0N, 10.8W)

3 Sep–5 Oct – W Cameroon(?)Juv 55–96 7140(05.6N, 09.9E)

(7200)(3 Sep–15 Oct,Total Juvn=2)

23 Aug–6 OctTotal average 6760

JOURNAL OF AVIAN BIOLOGY 32:1 (2001) 49

Table 2. Autumn migration of the six Ospreys that failed to complete their migration in 1996–1998. Approximate coordinatesare given for stopover sites and sites where signals were lost. Indices for the different individuals refer to the identificationnumber of the transmitter and the year it was used.

Sex/Age DistanceStart of Dates and sites of stopovers (coord.) Site and date ofInd.-Year flown, kmmigration disappearance (coordinates)

Fe 53–98a 8 Aug 10 Aug–15 Sep SW Norway (58.9N,06.1E)

260023–29 Sep Central Spain (40.4N, 04.5E) Central Spain (40.4N, 04.5E), 29Sep

0Ma 51–96 – Breeding area (59.6N, 15.4E), 23Aug

Juv 52–96 18 Aug 20 Aug–8 Sep SE Sweden (58.8N, 17.5E)180012 Sep–7 Oct N Slovakia (49.4N, 19.5E) N Slovakia (49.4N, 19.5E), 7 Oct

Juv 54–96 33029 Aug – SW Sweden (57.3N, 12.3W), 31 AugJuv 13–98 23 Aug 26 Aug–22 Sep N Poland (53.9N, 18.4E) N Poland (53.9N, 18.4E), 22 Sep 650Juv 48–98 3 Sep 7805 Sep–6 Oct W Norway (61.8N, 05.3E) W Norway (61.8N, 05.3E), 6 Oct

a Previously followed in 1996 (Fe 49–96, see Table 1).

parison with equatorial areas (Alerstam 1996). As abasis for evaluating topographical features along theflight routes and at stopover sites, the Times Atlas ofthe World was used (scale of map generally 1:2500000–5000000).

Mean and scatter of directional data were calculatedas mean vector directions, mean vector lengths (r) andangular deviations according to Batschelet (1981).Movements over a minimum distance of 100 km and aminimum time of 9 h were considered as segments(excluding irregular changes in positions duringstopover periods). The duty cycles of the transmittersand the availability of positions determined how manysegments the migratory journey could be divided intofor the different Ospreys. Only locations of high qualitywere used in this analysis.

Results

Migration routes

Of the 18 Ospreys provided with transmitters, six fe-males (Fig. 1), five males (Fig. 2) and two juveniles(Fig. 3) completed their autumn migration (with thepossible exception of Juv 55–96, see Table 1). FiveOspreys faced transmitter failure or died (Table 2). Oneadult male, who probably died, disappeared in thebreeding area in late August. His transmitter was lo-cated three km north of the nest, but the bird was notfound. Moreover, one adult female (Fe 49–96) was lostin Spain in September 1998, when being followed forthe second time (Table 2). Only two of six juvenilesreached Africa (Tables 1 and 2, Fig. 3). All of them leftthe breeding area, but the signals from four of themwere lost during the autumn. One was lost in southernSweden, shortly after departure from the nest. Theother three juveniles departed to Norway, Poland andSlovakia (Table 2, Fig. 3), where contact was lost afterextended stopover periods. The bird in Norway was

found moribund, and a veterinary X-ray showed dam-ages probably caused by collision with a power line.The cause of disappearance is not known for the otherthree, but it seems likely that they died. The juvenilewho reached Cameroon was killed by local people at asmall stream and the transmitter sent back to us by aFrench missionary. Assuming that all lost birds died,the mortality was close to significantly higher in juve-niles than in adults, although sample sizes were small(p=0.06, Fisher’s exact test).

All adults with satellite transmitters produced fledgedyoung, except Ma 55–97 (Table 1), whose breeding wasterminated in late July. The majority of the birdsmigrated across western Europe, and crossed the Med-iterranean Sea close to Gibraltar. However, two fe-males, one male and one juvenile migrated along moreeasterly routes, crossing the Mediterranean Sea eithervia Sardinia-Corsica (Fe 43–95 and Juv 55–96) orSicily-Malta (Fe 49–98 and Ma 51–98, Figs 1–3). Onebird (Fe 44–95) did not cross the Mediterranean Sea atall, but used a very easterly route across the Caucasusand the Red Sea (Fig. 1).

Most birds wintered in tropical West Africa, fromThe Gambia to the Ivory Coast, although one juvenile(55–96) reached Cameroon and one female (44–95)spent the winter in Mozambique (Table 1, Figs 1 and3). Despite the very easterly wintering site of this fe-male, there was a tendency, although not statisticallysignificant, that females more often wintered fartherwest than males (U=6, p=0.06, n1=5, n2=6, Mann-Whitney U-test). The total migration distance generallyvaried less than 1000 km around a mean of 6760 km(Table 1). The exception was Fe 44–95, who winteredin Mozambique and covered more than 10000 km.

Most of the birds made one or more stopovers(Tables 1 and 2). Of 21 recorded stopovers only fivewere south of 45°N; southern France, southern Spain,Tunisia, Ethiopia and Uganda. Generally, females leftthe breeding grounds before the males (cf. Kjellen et al.2001 for details) and flew to a stopover site, where they

50 JOURNAL OF AVIAN BIOLOGY 32:1 (2001)

remained stationary for 3–4 weeks before continuingmigration (Table 1). This also includes Fe 43–95, wholeft the male and two newly fledged young already on 3August, and then staged in an area with good fishinglakes about 30 km NW of the nest site (see Kjellen et al.1997). An interesting detail is that she subsequentlychose this area for nesting in 1996–2000 (pers. obs.). Fe48–96 probably also made a more extensive stopover(Table 1), but this could not be confirmed as thelocations provided on 15 through 21 August were ofinsufficient accuracy.

Most males and juveniles made stopovers on migra-tion. The exceptions were Ma 50–96 and Juv 55–96,who made no obvious stopovers at all. Ma 55–97, whosebreeding failed, behaved very similar to the females, ashe left the breeding area early and made an extendedstopover in east-central France (Table 1). Three of fivejuveniles that could be followed for more than two weeksafter departure seemed to adopt a similar strategy to thefemales, travelling to an initial stopover site not too faraway where they remained for a longer period (Table 2,Fig. 3).

After reaching Africa, adults seemed to travel directlyto a previously used wintering site, where they remainedfor the winter. The two juveniles were more mobile afterreaching the tropics. The one that reached Cameroontravelled more slowly and veered from a southerly to aneasterly course after arriving in Nigeria. The survivingjuvenile slowed down and made a sharp turn in a similarway before settling in Guinea (Fig. 3). In addition, itmade a number of movements of up to 480 km from themain wintering site during December–April.

Orientation

Adults generally seemed to keep a fairly straight direc-tion on the southbound migration, with only one indi-vidual turning back northwards. Ma 51–98 crossed theMediterranean Sea from Sicily via Malta to Libya, butthen flew northwest to make a 6-d stopover in northernTunisia before crossing the Sahara (Table 1, Fig. 2).Apart from the deviation by this male, two females(44–95 and 49–96) showed a high degree of directionaldeviation when leaving the nest sites for stopovers inKazakhstan and Norway, respectively (Fig. 1). Exclud-ing these deviations, directions generally varied between160 and 240° in all adults.

Some migration segments were apparently directedtowards specific stopover goals, like the male’s reversalin North Africa and the initial movements east andwest by the two females. Excluding these segments, themigration routes of the adult Ospreys were remarkablystraight with similar overall mean directions towards185–209°, and with mean angular deviations of 6–33°(corresponding to mean vector lengths r=0.84–0.99)(Table 3). Mean direction was 205° for females and200° for males. This difference is primarily caused bymore females wintering farther west. Female 44–95 didnot change the picture much as she, after the initialmovement to Kazakhstan, kept an average direction of195° when travelling to Mozambique.

Some juveniles initially migrated in directions clearlydeviating from the normal migration course (Fig. 3).The six juveniles provided with transmitters left thebreeding area in the directions 136, 165, 178, 192, 215

Fig. 1. Map (Mercator projection)of the migration routes of six femaleOspreys tracked by satellite from thebreeding site in Sweden to thewinter quarters in Africa.

JOURNAL OF AVIAN BIOLOGY 32:1 (2001) 51

Fig. 2. Map (Mercator projection)of the migration routes of five maleOspreys tracked by satellite from thebreeding site in Sweden to the winterquarters in Africa.

and 277°, respectively. The two juveniles that reachedAfrica showed a relatively large directional scatter alsoalong their migration routes (their variable directionsafter reaching the wintering latitudes were excludedfrom the calculations presented in Table 3) comparedto most adults. Mean directions for the two juvenileswere 189° and 195°, and angular deviations 24° and31°, respectively. Thus, there may be a tendency for thejuveniles to show more variable orientation than adults,which most often had deviations between 10 and 20°(Table 3).

If the Ospreys were guided by the principles of vectororientation, as discussed by Rabøl (1978), Mouritsen(1998) and Alerstam (2000), one would expect a re-duced directional scatter with increased segment length.However, there was no significant correlation betweenangular deviation and segment length, either when con-sidering all individuals (rs= −0.03, n=13, n.s., Spear-man rank correlation test) or when excluding thejuveniles (rs= −0.17, n=11, n.s.).

Discussion

Migration routes

Ringing recoveries show that the great majority ofSwedish Ospreys winter in West Africa south of theSahara. Before this study began, a total of more than15000 birds ringed, primarily as nestlings, had resultedin about 230 recoveries in western Africa compared toonly two in eastern Africa (O8 sterlof 1977, Stolt et al.

1998). Nevertheless, one of the birds in this studywintered in Mozambique, which suggests that caremust be taken when interpreting the results of ringingrecoveries. There seems to be no doubt, however, thatthe density of wintering Ospreys from north Europeanbreeding grounds is lower in eastern than in westernAfrica. The less frequent overall occurrence of winter-ing Ospreys in eastern Africa may possibly be due tothe risk of piracy from the African Fish Eagle Haliaee-tus 6ocifer, which is more numerous in this region (e.g.Brown et al. 1982).

In Senegal and The Gambia, most wintering Ospreysstay within very small areas, and a high proportionreturn to the same site in consecutive winters (Prevost1982). This study supports this pattern, as all our adultswent straight for a certain, presumably previously used,wintering site. Moreover, all the adults were very sta-tionary during the winter, generally moving less than 10km from the centre of the home range. Juvenilesseemed to search for a suitable wintering site at fairlylow speeds over rather large areas. This behaviourinvolved major changes in the general direction ofmigration when reaching the tropical region (Fig. 3).Young Ospreys probably arrive comparatively late inthe wintering area, only to find most good territoriesoccupied by adults. Juveniles do not return to thebreeding grounds during their second, and often noteven during their third calendar year (O8 sterlof 1977,Cramp and Simmons 1980). As indicated by the juve-nile staging in Guinea, it seems likely that they insteadmove around during this time, searching out a morefavourable site to use during forthcoming winters.

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During the migration, all except two of the birdsmade one or more stopovers, primarily in northern andcentral Europe. Most lakes in the breeding area atGrimso are oligotrophic and hold relatively small fishstocks compared to lakes in other areas in northernEurope (M. Hake unpubl.). Hence, although some ofthe birds obviously were able to prepare for migrationin the breeding area (cf. Kjellen et al. 2001), others mayhave found it favourable to use more profitable sitesnot too far from the breeding area before covering anylonger distances. These birds may have staged innortherly areas because they had previous experiencefrom visiting certain sites. This was apparently the casefor some of the females, but may also have been truefor some males, as indicated by Ma 55–97 (see below).Alternatively, good feeding sites were more easy to findat northerly latitudes.

Males generally used fewer days for stopovers thanfemales (Kjellen et al. 2001). The failed breeder (Ma55–97), however, adopted a strategy similar to thefemales. Possibly he was familiar with the stopover sitesused in 1997, as we know that he visited one of thesesites in France also the following autumn (observed byFrench ornithologists, Rolf Wahl pers. comm.). Thismale departed unusually late from his wintering site inspring 1998 (unpubl. data), and arrived in the breedingarea too late to breed. Thus, it seems that his autumnmigration behaviour in 1998 was the same as in 1997.Males whose breeding failed generally left the breedingarea earlier than successful males (Kjellen et al. 2001),which indicates that our males actually preferred amigration strategy similar to the one adopted by the

females. The Osprey shows a high degree of sexualdimorphism, with the female being larger than the male(e.g. Poole 1989). Consequently, females seem to domi-nate males. During breeding, males do most or allfishing and nest construction (e.g. Poole 1989, M. Hakeunpubl.). Therefore, males are forced to postponemoult until after the breeding season, whereas femalesnormally moult several flight-feathers while breeding(M. Hake unpubl.). As males care for the young afterthe females have departed, they may be forced to adopta less favourable migration strategy. Nevertheless, themales spent fewer days at stopover sites than the fe-males, particularly Ma 50–96, indicating that they werecapable of preparing for the southbound migration inthe breeding area.

Fe 43–95 left the nest already on 3 August (Table 1),and the positions up to 28 August indicated that sheprimarily frequented an area with good fishing lakesabout 30 km NW of the breeding site (Kjellen et al.1997). All other females moved considerably fartherbefore staging. Three of the females made stopoversmore or less en route south (Table 1, Fig. 1), whereastwo of the females moved to stopover sites well off themain route to Africa (Norway and Kazakhstan). Fe49–96 was ringed as a nestling in southern Norway(59.17N, 9.87E) in 1992 (Odd Frydenlund-Steen inlitt.), which indicates that she may have found a goodstopover site at the Stavanger Fiord in Norway duringdispersal in her first autumn. We supplied this femalewith a new transmitter in 1998, and she left for thesame area in Norway, spending over a month therebefore resuming migration south. Unfortunately she

Fig. 3. Map (Mercator projection)of the migration routes of twojuvenile Ospreys tracked by satellitefrom the breeding site in Sweden tothe winter quarters in Africa. Fouryoung birds that perished earlyduring autumn migration are alsoincluded on the map.

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Table 3. Mean and scatter of directions of Ospreys, based on directions of segments of their migratory routes from Sweden toAfrica, as recorded by satellite tracking.

Ind. No. of segments Angular deviation (degrees)Mean segment length (km) Mean direction (degrees) r

Fe 43–95 9 725 204 170.95Fe 44–95 12 830 29185 0.87Fe 44–95a 10 11748 195 0.98Fe 48–96 5 1163 6209 0.99Fe 49–96 5 1189 217 260.90Fe 49–96b 4 1363 19207 0.95Fe 51–97 22 17271 209 0.95Fe 49–98 14 489 33205 0.84Ma 50–96 5 171207 197 0.96Ma 55–97 8 733 17194 0.96Ma 51–98 24 33292 199 0.84Ma 51–98c 22 306 16197 0.96

25Ma 52–98 17 382 206 0.90Ma 54–98 11 570 9203 0.99Juv 53–96 8 31842 195 0.85Juv 55–96 9 24752 189 0.91

19Mean 11 727 201 0.94

a Excluding two initial segments towards Kazakhstan.b Excluding one initial segment towards Norway.c Excluding two segments towards W/NNE to stopover site in Tunisia.

was later lost in central Spain (Table 2). As the femalewho left for Kazakhstan flew directly there, it seemslikely that she too had previous information on herstopover site, possibly gained during dispersal as ajuvenile from a nest site in easterly regions. This issupported by the winter site chosen by this female, asthere seems to be a gradient in migration routes, withbirds breeding in more easterly regions wintering fur-ther east (O8 sterlof 1977, NOF 1988, Saurola 1994). Onthe other hand, female 49–98, which was ringed as anestling at Vaasa (62.60N, 24.30E) in W Finland in1993 (Finnish Ringing Centre in litt.), did not make anydetour during autumn migration. However, as she washatched about 600 km north of her later breeding area,this is not too surprising.

Orientation

Ospreys are strong flyers and therefore should hesitateless than other raptors before crossing geographicalobstacles, such as large bodies of open water or moun-tain ranges. This is supported by this study, as severalbirds flew over mountain ranges like the Alps, Cauca-sus and the Atlas mountains. Also, several sea cross-ings, e.g. of the Baltic, Mediterranean and Red Seas,were documented. This may explain why Ospreys areless concentrated at migration hot spots like Falsterboor Gibraltar compared to species that rely more onthermals during migration (Alerstam 1990a, Kjellen1997). Nevertheless, our birds did not seem to passextensive water areas at random points. Eight of thebirds crossed the Mediterranean Sea from SouthernSpain, although not necessarily at Gibraltar, and fourflew south via Italy (Figs 1–3). A similar behaviour was

recorded for Ospreys followed by satellite from Ger-many and USA (B. Meyburg pers. comm., M. Martellet al. unpubl.), indicating that Ospreys to some degreeavoid risks connected with the crossing of larger waterbodies.

Some juveniles departed in directions diverging fromthe general migration course (Fig. 3). Moreover, mostof them made long stopovers shortly after leaving thebreeding area. This may be an example of juveniledispersal, and also a demonstration of how birds mayfind stopover sites used later in life. It seems naturalthat orientation in young Ospreys, lacking previousexperience, should be less precise than that of adultbirds, which have made the trip before. This seems tobe the case for Swedish Honey Buzzards Pernisapi6orus, where juveniles show a considerably greaterscatter during autumn migration than adults, whichmainly proceed directly towards the Strait of Gibraltar(Søgaard and Østerby 1989, Stolt et al. 1992).

Autumn ringing recoveries show a pattern of wide-angle orientation, where not only juveniles but alsoadults from Sweden fan out widely across Europe on abroad-front migration en route to mainly West Africanwinter quarters (O8 sterlof 1977, Alerstam 1990a, b). Thisstudy indicates that postbreeding dispersal of juvenilesin various directions and goal-orientation by adult fe-males towards specific stopover sites (which may havebeen located and imprinted during the juvenile dispersalphase) may be the most important reasons for the widescatter of ringing recoveries of Ospreys during theautumn. Other segments of the migratory routes areremarkably straight and concentrated around a meandirection towards SSW (Table 3).

While the magnetic declination is close to 0° inSweden, it is about 10°W in the main West African

54 JOURNAL OF AVIAN BIOLOGY 32:1 (2001)

winter quarters (Senegal-Ivory Coast). Consequently, ifthe Ospreys oriented along a constant magnetic com-pass course, their geographic direction is expected toshift gradually by about ten degrees towards the leftthroughout their autumn migratory journey. There isno such tendency of a leftward course change, makingorientation along magnetic loxodromes unlikely.

Rabøl (1978) and Mouritsen (1998) demonstratedhow migrants drawing their orientation during eachflight step from a circular probability distribution witha fairly large scatter will, by the effect of vector summa-tion over many flight steps, produce resulting long-dis-tance migratory directions with a much more restrictedangular scatter. With a directional concentration ofeach step corresponding to a mean vector length r, thebird would follow a circuitous route on average 1/rtimes longer than the straight-line distance to the desti-nation (cf. Alerstam 2000). One day’s movement over adistance of 200–400 km (Kjellen et al. 2001) mayrepresent an orientational vector step for the Osprey.Table 3 indicates that the directional concentrationbetween steps of such distances (recorded for threeadults) corresponds to r=0.90–0.96. For the two juve-niles, a corresponding directional step concentration ofr=0.7–0.8 may be provisionally inferred from Table 3(assuming that their mean segment length correspondsto three orientation steps), in conjunction with theappendix table in Mouritsen (1998). For a migratoryjourney requiring n steps (= travelling days) with ex-actly straight-line orientation, juvenile Ospreys would,with the orientation precision between steps as indi-cated above, use 1.2n to 1.4n steps to complete thejourney, while adult Ospreys would complete it by1.04n to 1.1n steps. These estimates remain provisionaland uncertain given the limited data available and thelack of a distinct negative correlation between direc-tional scatter and segment length that is expected fromthe vector summation model (Mouritsen 1998, Aler-stam 2000).

Acknowledgements – We thank Katarina Hake and ClaesAxang for help with catching Ospreys. Martin Green kindlyhelped us with the maps. The study was supported by Crafo-ordska Stiftelsen (grants to N. Kjellen), the Swedish NaturalScience Research Council (to T. Alerstam) and MagnusBergvalls Stiftelse, Lunds Djurskyddsfond, Langmanska Kul-turfonden and Olle och Signhild Engkvists Stiftelse (grants toM. Hake). The study was approved by the Ethical Committeefor Research on Animals, Goteborg (permit 164/95 to M.Hake). Reto Spaar and Felix Liechti gave valuable commentsas referees.

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(Recei6ed 28 October 1999, re6ised 15 February 2000, accepted11 April 2000.)

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